Embryos recovered from random-source domestic cats between 1 and 7 days after ovulation and unfertilized oocytes recovered within 6 hours of ovulation were examined ultrastructurally for the presence of types A and C viruses. Intracisternal type A particles were found consistently within cells of the inner cell mass in blastocysts but not within trophoblast cells or cells from earlier stages of development. Mature type C viruses were not observed. Novel virus-like particles were found within cytoplasmic cisternae in many of the embryos; these particles did not appear to be associated with a particular stage of embryonic development.